Oxide barriers are an important component of superconducting and magnetic devices based on junctions utilizing electron tunneling. Uniformity of the tunnel barrier is crucial in obtaining reproducible tunnel junctions for use as memory elements in MRAM (Magnetoresistive random access memory) devices and in superconducting electronics based on Josephson and other effects. A component of such devices is an electron tunnel barrier, generally formed by oxidizing a metal. An oxide of zirconium, ZrOx, has been experimented with as a barrier material. The technique by which it was formed did not allow one to obtain high quality barriers that are free of pinholes, where the latter degrade device performance by producing a leakage current. Typically, a few nanometer thick overlayer of Zr is deposited in a single step, followed by oxidization to form an insulating barrier. With this approach the initial Zr overlayer turns out to be polycrystalline, and when oxidized it produces an inhomogeneous oxide; the resultant junctions then have a high leakage current. To overcome this drawback and to have the overlayer amorphous, some workers have alloyed Zr with another metal such as Al. This technique has the disadvantage of introducing into the multilayer structure a “soft” metal with a low melting temperature, and the need to precisely control the alloy composition for reproducibility.
In particular, At present, the most popular material used in tunnel barriers is AlOx; however it is based on a “soft” metal Al, which poses problems for the formation of robust, high-quality junctions based on refractory metals or other electrode materials that require high-temperature processing. There is a similar problem with MgOx barriers, which have also been tried as tunnel junctions. Zr forms a thermally stable oxide. ZrO2, which melts about 3000° C. Zr overlayers (deposited to form artificial barriers [1-3]) are known to be polycrystalline, so that, on being thermally oxidized, they produce a spatially inhomogeneous oxide, ZrOx [4], and the resultant junctions have high leakage currents. This property is a considerable drawback for using ZrOx in practical tunnel junctions. Proposals to improve the Zr barrier quality include the addition some other metals, such as Al, as an overlayer on the Zr [4-10]; however, it is understood that high-quality junctions have not been demonstrated within this approach.
Moreover, multiply oxidized Al barriers were reported for both superconducting and magnetic junctions [12-16]; however, it is understood that there has not been any publication on multiply oxidized Zr-based barriers.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.